1. Field of the Invention
This invention is in the field of capstock for plastics. More particularly, this invention relates to acrylic resin compositions which are especially useful as capstock materials for coextrusion over or lamination to structural plastics, especially polystyrene, as well as to the manufacture of such composites and to the articles produced therefrom.
2. Prior Art
Certain structural plastics, such as high impact polystyrene (HIPS), acrylonitrile/butadiene/styrene (ABS) resins, poly(vinyl chloride) (PVC) resins, and the like, exhibit attractive mechanical properties when extruded, molded, or formed into various articles of manufacture. Such articles include, for example, bathtubs, shower stalls, counters, appliance housings and liners, building materials, such as doors, windows and shutters, and storage facilities. Although these structural plastics are strong, tough and relatively inexpensive, the properties of their exposed surfaces are less than ideal. That is, the surfaces of the structural plastics are degraded by light; they can be easily scratched; they are eroded by common solvents, etc.
Consequently, it has become a practice in the industry to apply another resinous material over the structural plastic to protect the underlying structural material and provide a surface that can withstand abuse associated with the use environment. Such surfacing materials are called “capstocks”.
The capstock generally is much thinner than the structural plastic, typically being about 10 to about 25% of the total thickness of the composite comprising the capstock and structural plastic plies. For example, the thickness of the capstock can be about 0.1 to about 2.5 mm, whereas the thickness of the structural plastic ply can be about 1.0 to about 10 mm.
As a class, acrylic resins, known for their excellent optical characteristics, resistance to degradation by sunlight, hardness, inertness to water and common chemicals, durability, and toughness, are capstocks of choice for various structural plastics, such as ABS sheet. The mechanical properties of the capstock generally are secondary to those of the structural plastic, but it is important that the capstock not adversely affect the mechanical properties of the composite.
A resinous capstock, including one comprising an acrylic resin, can be applied to a structural plastic in several different ways. For example, preformed sheets or films of the structural plastic and the capstock can be laminated together, as by thermal fusion, by press lamination, or by lamination via an appropriate adhesive or via a mutually compatible polymer interlayer.
Other methods of lamination, such as co-calendering or bi-extrusion or even solution or dispersion casting, can be used to laminate structural plastics and an acrylic capstock. Alternatively, in appropriate cases, the structural plastic and an acrylic capstock can be coextruded, particularly feedblock coextruded, and this is often a choice method when the laminate is an acrylic-capped ABS sheet. A sheet of the composite can then be thermoformed into an article such as a bathtub, a shower stall, a counter top, etc.
Equipment for such coextrusion is widely available in the industry. The term “feedblock coextrusion,” as used herein, refers to a process in which each of a resin supply of a first plastic material (for example, a structural plastic material) and a resin supply of a second plastic material (for example, a capstock material) is heated to a molten condition as separated streams thereof are fed to a feedblock in which the streams are brought together in face to face surface contact to form a two-ply stream which in turn is fed to a sheet die in which the two-ply stream is spread laterally into a two-ply sheet under conditions of laminar flow, with the molten resin plies of the sheet being in face to face surface contact as they emerge from the die, the shaped two-ply sheet thereafter being cooled and solidified, the resultant composite comprising each of the plies of the sheet integrally bonded to each other by the solidified resins comprising the sheet. As is known in the art, the process of feedblock coextrusion can be used to produce a sheet of more than two plies by use of a feedblock which is designed to accommodate more than two streams of resin. In a process for forming a two- or higher-ply sheet, the edges of the sheet are usually trimmed, and ideally, the trimmings are recycled into the structural plastic feed to conserve material and realize economies.
A condition for use is that the capstock must have excellent adhesion to the structural plastic. However, in current acrylic resin capstock over, for example, High Impact PolyStyrene (HIPS) films or sheet articles, adhesion is unsatisfactory for commercial applications. To achieve a desirable or improved level of adhesion between the two layers, a third polymer is typically extruded in between the acrylic and HIPS, the so-called adhesive layer. An example of an adhesive layer is a MMA/styrene copolymer that has compatibility and/or miscibility with PMMA and HIPS. Prior art reference U.S. Pat. No. 4,350,742 discloses an alternative way to improve the adhesion between an acrylic and styrenic layer, co-polymerized 3-30 weight percent of α, β-unsaturated carboxylic acid monomer with the styrene to obtain good adhesion to the acrylic resin. The use of the intermediate layer results in additional processing steps, costs and weight to the composite. It is therefore a primary object of the invention to provide a coextruded product comprised of two layers, the first layer comprised of a structural plastic, preferably, high impact polystyrene, and a second layer being comprised of (a) an acrylate resin and (b) an acrylic polymeric additive that increases the adhesive strength between the layers.
A specific object of the invention lies in the production of a multiple-layered sheet comprised primarily of polystyrene and having a protective surface layer of an acrylic capstock.
Other objects, features and advantages of this invention will become apparent from the description which follows.